1. Field of the Invention
The present invention relates to medical scissors having handle parts and blades.
Medical scissors normally consist of high-alloy stainless steel (medical steel). The scissors are hardened and polished so as to guarantee good cutting properties over an extended period of time.
Good cutting properties require, however, a specific contact pressure between the halves of the scissors. During the cutting operation as such, the blades move about a substantially point-shaped pivot area relative one to the other. This means that the cutting force is concentrated substantially at the substantially point-shaped crossing area of the two blades that move relative one to the other.
Due to the relatively high friction factor of the steel grades usually employed, a certain tightness, with corresponding wear, of the halves of the scissors, predominantly in the blade area, cannot be avoided. The sharp condition of the medical scissors is therefore limited, and the scissors loose their optimum cutting behavior rapidly.
Especially high demands are placed on the material of medical scissors because these are regularly sterilized with super-heated water vapor at high temperatures. The scissors therefore have to be corrosion-resistant.
In addition, the scissors must exhibit a high degree of biological inertness, as even slightest traces of abrasion of the blades may trigger allergic reactions with sensitive persons.
In addition, the cut as such should be as clean as possible, i.e. it should be applied atraumatically.
Even a slight contusion of tissue leads to increased bleeding when separating tissue parts in live bodies.
Increased bleeding delays the healing process, whereas clean edges of a wound will unite more rapidly.
2. Related Prior Art
DE 42 35 023 A1 describes a gripping and/or cutting instrument for endoscopic purposes, that can be guided precisely and permits extremely clean cuts to be made without the surrounding tissue being damaged. It is proposed for this purpose to provide certain components with a friction-reducing material, such as polytetrafluoroethylene.
From DE 42 12 053 C1 a surgical instrument, made from metal, has been known for thermally cutting and/or coagulating biological tissue, where the metallic surface is coated, at least in part, with a hard substance. The hard substance coating consists of one metal-metalloid compound, or a mixture of metal-metalloid compounds, where the metal is one from the fourth to eighth side-groups or an element of the third main group of the periodic system of the elements, and where the metalloid is selected from the group containing nitrogen, carbon, oxygen and boron. The hard substance layer may consist of (TiNb)ON, containing 41 atom percent of titanium, 19 atom percent of niobium, 31 atom percent of nitrogen and 9 atom percent of hydrogen. The precious-metal layer may have a thickness of between 0.01 and 3 micrometers.
Prom U.S. Pat. No. 5,507,760 a cutting tool for introduction into a catheter has been known, having a base that may be made from steel selected from the group of the 440 FSe and 440 C steels. The cutting edge exhibits a low-friction and hardening layer selected from diamond-like carbon, aluminium oxide, titanium nitride, titanium carbonitride, zircon nitride, boron nitride, cubic boron nitride and high-chromium compounds.
U.S. Pat. No. 5,152,774 describes medical scissors where a base of stainless steel or titanium is coated with a nitride layer by means of a VD process. The nitride layer is selected from the group consisting of titanium nitride, titanium nitride alloys and zirconium nitride, that are deposited at temperatures of 175 to 225xc2x0 Celsius. The nitrided portion of the instrument has a Rockwell hardness xe2x80x9cCxe2x80x9d of at least approximately 50.
In the case of the known scissors mentioned above, the requirements previously mentioned are not fulfilled satisfactorily.
Thus, it is the object of the present invention to improve scissors of the before-mentioned kind in such a way that they will move easily, i.e. cut with little friction, that they will cut atraumatically, i.e. produce clean, rapidly healing cuts, and that are biologically inert and corrosion-proof.
This object is achieved according to the invention by the fact that the scissors are coated, at least in the area of the cutting edges, with an amorphous thin layer containing silicon, carbon and hydrogen, where the proportion of silicon is up to 100% atom parts in the boundary layer to the metallic body of the scissors, and up to 30% atom parts in the area of the outer surface of the layer.
With this configuration and a gradient in the silicon content and in the carbon and hydrogen as the other components, respectively, it is possible to achieve a coating which is extremely smooth and very hard and, in addition, chemically and biologically inert.
The very smooth layer is achieved by the fact that the layer is precipitated as amorphous layer. A crystalline layer would be considerably rougher due to the formation of crystallites.
The amorphous smooth layer allows low-friction and easy cutting, and the hardness of the layer produces in addition a wear-reducing effect.
Thus, it is possible with scissors coated in this way to make atraumatic cuts, and this permanently, as will appear from the test to be described later, where 10,000 cuts were made with a pair of scissors according to the invention without any variation, whereas in the case of conventional scissors increasing abrasion could be observed already after 500 cuts.
Due to the gradient in the silicon content according to which the latter is very high at the boundary surface to the base of the scissors, it is possible to achieve excellent binding of the layer to the base of the scissors. The declining silicon content and the resulting increasing content of carbon and/or hydrogen in the layer near its outer surface leads to an extremely hard, smooth and amorphous structure that presents the properties sought. The amorphous layer, consisting of silicon, carbon and hydrogen, is chemically and biologically inert, which means that there is no risk of corrosion of the layer, neither by chemical substances nor by biological attacks, if any.
The extreme hardness and smoothness of the outer surface of the layer can be explained by the fact that while sp3 hybrid structures, i.e. the basis for a diamond-like lattice, are present in the carbon and the silicon as well, the formation of crystals is obviously prevented by the hydrogen as xe2x80x9cinterfering substancexe2x80x9d.
The object of the invention is thus perfectly achieved.
The content of silicon at the boundary layer amounts preferably to up to 95% atom parts, most preferably to 10 to 90% atom parts.
The term xe2x80x9cpercent atom partsxe2x80x9d is used to describe the number of specific atoms, related to the total of 100 atoms.
It is further preferred that the content of silicon in the area of the surface amounts to 0 to 30% atom parts, and that at the same time the content of hydrogen in the layer amounts to 10 to 50% atom parts, the rest being carbon.
Within this variation range, it is possible on the one hand to achieve very good binding of the layer to the metallic base of the scissors and, on the other hand, to produce an extremely smooth, amorphous and hard layer on the outside, due to the higher carbon content. In both areas usual coating methods can be used by which the three components silicon, carbon and hydrogen can be precipitated in varying proportions.
Preferable, the thickness of the layer is 0.5 to 5 xcexcm, or most preferably 0.5 to 2 xcexcm.
These low thicknesses of the layer already permit to achieve excellent results so that medical scissors can be coated according to the invention with an economically reasonable input of material and equipment.
Most preferably, the layer consisting of silicon, carbon and hydrogen is precipitated by the PVD process (physical vapor deposition) and/or the CVD process (chemical vapor deposition), especially by the PECVD process (plasma-enhanced chemical vapor deposition).
By using these precipitation methods it is possible in an easy way to produce the desired layer with the desired gradient in silicon content.
If, for example, it is desired to apply exclusively silicon at the boundary surface to the base of the scissors, this can be effected by means of a PVD process, for example by what is known as sputtering. Small hydrogen contents can be realized by precipitating amorphous Si:H by a PECVD process, which makes use for example of silicon/hydrogen compounds. The subsequent deposits with a content of carbon can be produced especially by PECVD methods using compounds containing silicon, carbon and hydrogen, for example silicon/methyl compounds. The higher proportion of carbon may be achieved by introducing gradually into a PECVD process compounds richer in carbon, such a hydrocarbons, so that the layer can be built up with the desired gradient in a defined way and with smooth transitions.
Particular preference is given to a PECVD process where initially the halves of the scissors are cleaned from a possible oxide layer in an inert gas plasma, whereafter the corresponding gases are added for applying the inventive layer of silicon, carbon and hydrogen on the now extremely fine base of the scissors.
It is understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the context of the present invention.